Elsevier

Catalysis Today

Volumes 133–135, April–June 2008, Pages 699-705
Catalysis Today

Citral hydrogenation over Ir/TiO2 and Ir/TiO2/SiO2 catalysts

https://doi.org/10.1016/j.cattod.2007.12.109Get rights and content

Abstract

Catalytic hydrogenation of citral over iridium supported on TiO2, SiO2 and mixed oxides TiO2/SiO2 has been studied. The effect of the reduction temperatures (473 or 773 K) and the successive enrichment of TiO2 were analyzed. The solids were characterized by H2 chemisorption at room temperature, N2 adsorption at 77 K, XRD, TEM, FTIR and XPS. The obtained results show that the high temperature reduction treatment (HT) produces an important enhancement in both the catalytic activity and selectivity towards the desired product. The behavior is explained on the basis of a surface enrichment in titania due to the migration of TiOx species on the Ir crystallites, which does not occur in those catalysts reduced at low temperature (LT) or in the catalyst supported on an inert oxide such as SiO2.

Introduction

Citral hydrogenation is a process of increasing interest within the set of reactions of selective hydrogenation of α, β-unsaturated aldehydes especially important in the field of fine chemistry [1]. The control of the selectivity by means of the hydrogenation of Cdouble bondO bond without altering Cdouble bondC bond is required to obtain the unsaturated alcohol. Supported noble metal catalysts on inert supports showed generally a low selectivity towards the reduction of carbonyl group and consequently some efforts are realized through the nature of the active metal, support effects, the addition of promoters and metal particle size, among others [2], [3]. It has been reported that Ir/TiO2 can be very active and selective towards the hydrogenation of carbonyl bond during citral hydrogenation [4]. The significant enhancement in the selectivity is explained by the SMSI effect (strong metal support interaction); reduction at high temperature leads to surface decoration of the iridium metal crystallites by TiOx species, which allows to the polarization of carbonyl group.

High surface area of titania is desirable for these applications, however, a great disadvantage of titania as a support is its rather low surface area. Inert oxides such as silica have been used as carrier to deposit titania in order to increase the area of this partially reducible oxide. The morphology as well as the surface properties of the TiO2−x coated supports depend on the preparation procedure, often grafting methods are used to obtain this type of solids. Its preparation requires the binding of a highly reactive titanium precursor, such as TiCl4 or alkoxide organic to the silanol groups of the silica surface [5], [6], [7], [8].

There are two types of Ti species in these supported oxides: segregated TiOx, and isolated Ti species that interact with Si-OH groups [6], [7]. It has been observed that the Ti(i-OPr)4 may reacts with a silica surface and after a calcinations leads to highly dispersed TiO2 [10], [11], [12], [15] with strong interactions among the two oxides as a consequence of Tisingle bondOsingle bondSi bonds [15]. Capel-Sánchez et al. [8], using UV–vis spectroscopy, DRIFTS and XPS, have studied the chemical environment of titanium in these supported oxides. The results have demonstrated changes in the molecular structure of titanium species, most of these materials showed varying proportions of tetrahedral and octahedral titanium species.

On the other hand, the pretreatment temperature, the reaction temperature, and the molecular size and reactivity of the precursors and taking in account the different positions of silanols on silica [9], [10], [11], each titanium center can be bonded to one, two or even three silicon atoms through Sisingle bondOsingle bondTi bonds, leading to the formation of monopodal, bipodal, or tripodal species, respectively [10]. High hydroxyl group concentration and low temperatures (>373 K) are preferred in order to prepare high surface coverage of molecularly dispersed TiO2/SiO2, The porosity and morphology of the silica can also influence an efficient grafting. Srinivasan et al. [5] have reported a maximum dispersion of TiO2 at ∼3.0 Ti atom/nm2 on a non-porous SiO2 (Stöber silica spheres) due to its highest surface hydroxyls concentration and highest accessibility to the reagent. Gao and Wachs [12] claimed that the dispersion could be increased up to 4.0 Ti atom/nm2 by a careful control of the preparation variables. Anchored titanium should form a monolayer on silica only for TiO2 loading in the range 15–20 wt.%, depending on the silica surface area [13]. At higher loading crystalline phase of rutile or anatase can be observed.

Binary oxides have been used as catalysts and supports for a wide variety of reactions [9], [10], [14], [15], [16]; in addition, SMSI effect ascribed to TiO2 in this type of solids when it has been used as catalytic support. Kumbhar [14] have studied Ni/SiO2-TiO2 catalysts showed strong suppression in hydrogen chemisorption capacity similar to a Ni/TiO2 catalyst indicating strong metal support interaction, this latter catalyst showed good activity for the liquid phase hydrogenation of acetophenone. However, information about the use of this mixed oxide as support of metal catalysts for the selective hydrogenation in α, β-unsaturated aldehydes has not been reported. Hoffmann et al. [15], have studied the metal–support interaction by following the shift in the band due to the carbon monoxide adsorbed on platinum dispersed on silica modified with titania; this effect was only observed on Pt/TiO2/SiO2 catalyst with the highest titanium content and a well defined anatase crystalline phase, being previously submitted to high temperature reduction treatment (HT 773 K).

In this work, the hydrogenation of citral over Ir/TiO2/SiO2 (Ir/G) catalysts was studied. The main objective was to study the catalytic behavior of Ir/TiO2, Ir/SiO2 and Ir supported in mixed oxides (Ir/G) reduced to 473 and 773 K, in order to analyze the effect of the partially reducible support, the successive enrichment of titania and the reduction temperatures of the catalysts. The solids were characterized by the following methods: H2 chemisorption at room temperature, N2 adsorption at 77 K, XRD, TEM, and X photoelectron spectroscopy (XPS). The liquid phase hydrogenation of citral was studied in a batch reactor at 0.62 MPa of H2 and 363 K of reaction temperature.

Section snippets

Synthesis of Ir/TiO2 and Ir/SiO2

Ir/TiO2 and Ir/SiO2 catalysts were prepared by impregnation at 313 K of TiO2 (Degussa P-25, SBET = 70 m2 g−1) and silica (Syloid-266-Grace Davidson, SBET = 290 m2 g−1) with an aqueous solution of H2IrCl6 to give an Ir loading of 1 wt.%. The impregnated solids were dried at 373 K for 6 h, calcined in air at 673 K for 4 h and reduced at 473 K (LT) and 773 K (HT) for 2 h in flowing hydrogen.

Synthesis of Ir/TiO2/SiO2 (Ir/G)

Titanium isopropoxide (Aldrich, reagent grade) (1 mmol g−1 of silica) was dispersed in toluene (150 ml) and added to an Aerosil

Catalysts characterization

Table 1 summarizes the results of TiO2 content, specific surface area and H/Ir ratios and the Ir particle size from evaluated TEM of supported Ir catalysts. The porosity of SiO2 allows that Ir/SiO2 presents a specific BET surface area larger than Ir/TiO2. Specific surface of the Ir/G catalysts decreases with the increase in TiO2 content, which is explained as a gradual coverage of the SiO2 by TiO2, similar results were found by Castillo et al. [7]. The H/Ir ratio is relatively low for the

Conclusions

The results show that the TiO2 loading and the reduction treatment plays an important role on the catalytic behavior of Ir/TiO2/SiO2 catalysts. The reduction temperature produces significant differences in the activity, thus, at low reduction temperature, the solids exhibit low activity without significant differences in each series. The addition of increasing amount of Ti to the SiO2 support increases the surface coverage making easier the interaction with the metallic component which is

Acknowledgement

The authors would like to thank UPTC-COLCIENCIAS for financially supporting this research under contract No. 110951717865.

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